1. Field of the Invention
The present invention relates to a new and improved electrode for electrochemical sensors and to a new and improved method of fabricating electrodes for electrochemical sensors; and more specifically, to a new and improved disposable, inexpensive electrode for electrochemical sensors and a method of fabricating the electrode.
2. Description of the Prior Art
Products that measure fluctuations in a person's blood sugar, or glucose levels have become everyday necessities for many of the nation's seven million diabetics. Because this disorder can cause dangerous anomalies in blood chemistry and is believed to be a contributor to vision loss and kidney failure, most diabetics need to test themselves periodically and adjust their glucose count accordingly, usually with insulin injections. Patients who are insulin dependent--about 10% to 15% of diabetics--are instructed by doctors to check their blood-sugar levels as often as four times daily.
For years the solution for diabetics was one of several urinanalysis kits that, despite repeated improvements, provided imprecise measurements of glucose in the blood. The first such kits used tablets. This early testing procedure is described in U.S. Pat. Nos. 2,387,244 and 3,164,534. Later, reagent strips for urine testing were developed. Testing using urine, however, is limited in accuracy. The renal threshhold for glucose spillage is different for each individual. Moreover, sugar in urine is a sign that the sugar level is too high and has been too high for several hours. This is due to the delay in sugar reaching the urine.
More accurate readings are possible by taking readings from blood. The advent of home blood tests is considered by some to be the most significant advance in the care of diabetics since the discovery of insulin in 1921. Home blood glucose testing was made available with the development of reagent strips for whole blood testing. Reagent strips of this type are described in U.S. Pat. Nos. 3,164,534 and 3,092,465. A breakthrough in self-care came in 1979, when the Ames division of Miles Laboratories brought out its Visidex home blood test. Visidex consists of chemically coated plastic strips. When blood drawn by pricking a finger is placed on one of these disposable strips, the resulting color change can be compared with a color-coded glucose scale included in the kits or a reflectometer can be used.
The advantage of the current technology available for home use, the reagent strip, is low cost (roughly fifty cents per use) and a one minute response time. There are significant problems with reagent strips, however. Test timing using reagent strips is very critical. Exactly sixty seconds must elapse from the time a blood sample is placed on a strip to when it is removed by rinsing. The color on the strip must then be compared with a chart. This time constraint and the necessity to ascertain differences in shades of colors results in the technique being very user sensitive.
An alternative to reagent strips is a glucose sensor using an electrode. Electrodes are more costly and the technology is more complicated but the life of an electrode is weeks or months as compared to the single use of a reagent strip. The response time of electrodes is quick and electrodes are not user sensitive resulting in increased accuracy over reagent strips.
Electrodes in electrochemical glucose sensors utilize an enzyme to convert glucose to an electroactive product which is then analyzed electrochemically. The reactions for this electrode are given in the following equations: EQU C.sub.6 H.sub.12 O.sub.6 +O.sub.2 +H.sub.2 O=C.sub.6 H.sub.12 O.sub.7 +H.sub.2 O.sub.2 EQU H.sub.2 O.sub.2 =O.sub.2 +2H.sup.+ +2e.sup.- EQU O.sub.2 +4H.sup.+ =2H.sub.2 O
In the first equation glucose is oxidized by oxygen to form gluconolactone and hydrogen peroxide. This reaction is catalyzed by the enzyme glucose oxidase. The hydrogen peroxide may be detected either by oxidation as shown in the second equation or by measuring the decrease in oxygen partial pressure by the reaction shown in the third equation. In either case a current is obtained which is related to the glucose concentration. The oxidation of the hydrogen peroxide is done at a platinum electrode and the reduction may be done at either a platinum or a silver electrode.
In these electrodes, diffusion of glucose through membranes and reactions of glucose in membranes is of concern. In known electrodes, glucose and oxygen from diluted blood as well as many interferents diffuse through a primary membrane. As glucose diffusing through this membrane reaches a second membrane, glucose oxidase catalyzes the conversion of the glucose to hydrogen peroxide and gluconolactone. The hydrogen peroxide may diffuse back through the primary membrane or it may further diffuse through the second membrane to the electrode where it can be oxidized back to oxygen and produce a current used for analysis. The secondary membrane prevents passage to the electrode of substantially everything except the hydrogen peroxide.
Glucose electrochemical sensors are essentially made up of two major components; a permanent or factory replaceable electrode and a user replaceable, disposable membrane assembly including a primary membrane and a secondary membrane. The electrode is based on a Clark electrode operating in the hydrogen peroxide mode. An electrode of this type is described in U.S. Pat. No. 2,913,386. The Clark electrode includes a platinum anode and a silver cathode. A voltage of 0.7 volts is applied to the electrode and current between the cathode and anode is measured.
The primary membrane is used to separate high molecular and cellular components of the blood from the glucose. This membrane must be permeable to glucose but relatively impermeable to the larger molecular and cellular components of blood. The typical primary membrane is not whole blood compatible, since to do so requires a surface treatment. Due its this incompatibility, the primary membrane is quickly fouled by protein deposits or blood clots requiring the membrane assembly to be replaced by the user.
Before electrochemical sensors can be made for use in the home, technology must be advanced to allow measurements using whole blood. This has not been achievable in electrochemical sensors to date because primary membranes presently used are not whole blood compatible and are quickly fouled by contaminants in whole blood. Many of the disadvantages of prior sensors could be eliminated if a longer lasting membrane could be provided. A membrane of this type is disclosed in copending application Serial No. 749,724 filed concurrently with the present application and assigned to the assignee of the present invention. A membrane of this type, however, is bonded to the electrode. When the membrane fails, the membrane and electrode must be replaced rather than only the membrane, and it is desirable to provide an inexpensive electrode that can be disposed of after several uses.